The invention relates generally to optical temperature measurement, and more particularly to optical temperature measurement based on the temperature-dependent absorption and fluorescent emission properties of selected materials.
Temperature sensing and control are crucial in a variety of situations arising in medicine, industrial operations, and scientific research. In many cases, temperature measurements must be conducted remotely because the processes or machinery to be monitored is inaccessible or involves hazardous components, such as, high pressures, high radiation levels, high intensity magnetic or electrical fields, corrosive materials, or the like.
In medicine the need for temperature monitoring arises in several contexts. Cardiac output and blood flow rates are measured by the thermal dilution method, wherein a bolus of room temperature solution is injected into a vein and blood temperature changes are monitored by one or more temperature sensing devices located down stream. For accurate measurements, it is desirable to use sensors with frequency responses greater than the major frequency components of the arterial pressure pulse, i.e., faster than about 20-30 Hz. In operations involving artificial hypothermia, such as heart and neurological cases, temperature monitoring is critical. Accurate temperature monitoring is also critical where local hyperthermia is induced as a means of cancer therapy. The differences between normal cells and malignant cells in their sensitivities to thermal killing is often no more than a few tenths of a degree. The presence of metallic sensors greatly complicates temperature control in this mode of therapy as the presence of such sensors can alter the thermal characteristics of the tissue in which they are imbedded. Furthermore, electromagnetic heating of tissues causes special difficulties: electromagnetic interference is induced in the thermometry electronics, excessive and artifactual heating occurs in sensors constructed of resistive material (both thermistors and themocouples), and sensors, especially those contained in highly conductive (and hence reflective) shields, perturb the electromagnetic fields used for heating. Thus, application of metallic, electrically-based temperature sensors presents significant problems: reduced accuracy due to noise and inadequate frequency response, and direct electrical hazard to the patient, especially when more than one sensor is employed.
In industrial process control the most common techniques for temperature measurement utilize thermocouples, thermistors, and resistance thermometers. These devices generate electrical signals which are amplified and then converted into temperature readings or employed in control functions. Frequently, these devices are impractical because the process is inaccessible, too hazardous, or too corrosive for in situ placement of sensors. For example, temperature monitoring of nuclear reactor vessels and coolant systems, underground nuclear waste-disposal sites, chemical dumping sites, working zones of coal-liquification reactors, oil refinery processes, geothermal wells, and like processes, all involve conditions which make the use of standard electrically-based sensors difficult or impractical.
Many of the above-mentioned difficulties with current information-gathering technology can be overcome by using remote, in situ optical probes coupled to a detector by optical waveguides, or fiber optics. Fiber optics are durable, corrosion-resistant, heat-resistant, impervious to electrical or magnetic interference, and are available in very small diameters, which makes them amenable for use with miniature probes.
A large variety of optical temperature sensors have been developed, some of which are amenable for use with fiber optics, Wickersheim and Alves, "Recent Advances in Optical Temperature Measurement," Industrial Research and Development, December 1979; Peterson and Vurek, "Fiber-Optic Sensors for Biomedical Applications," Science, Vol. 224, pgs. 123-127 (1984). Optically-based sensors generating fluorescent signals modulated by ambient conditions are particularly well suited for use with optical fibers, and several temperature sensors have been developed which are based on the temperature dependent fluorescent properties of selected materials, e.g. Quick et al., U.S. Pat. No. 4,223,226, issued September 1980, entitled "Fiber Optic Temperature Sensor,"Samulski, U.S. Pat. No. 4,245,507, issued Jan. 20, 1981, entitled "Temperature Probe," Samulski, U.S. Pat. No. 4,437,772, issued Mar. 20, 1984, entitled "Luminescent Decay Time Techniques for Temperature Measurement;" and Hirschfelo, U.S. Pat. No. 4,542,987, issued Sept. 24, 1985, entitled "Temperature-Sensitive Optrode;" and Haugen and Hirschfeld, "An Ultrafast Remote Sensor for High Pressures and Temperatures," Energy and Technology Review, pgs. 78-79 (July 1985). Sensitivity is a major problem with this class of sensors, particularly when used with fiber optics of significant length. That is, it is difficult to obtain a strong enough signal from a fluorescent probe to permit the measurement of small differences in temperature.
Of particular interest are U.S. Pat. Nos. 4,075,493 and 4,215,275 by Wickersheim, issued 21 Feb. 1978 and 29 July 1980, respectively, and both entitled "Optical Temperature Measurement Technique Utilizing Phosphors." Wickersheim discloses a method of measuring temperature by monitoring the intensity ratio of at least two distinct and optically isolatable fluorescent emission lines of selected rare earth-doped compounds. His invention requires at least two photodetection devices, one for each emission line.
Also of interest is Snitzer et al., U.S. Pat. No. 4,302,970, issued 1 Dec. 1981, entitled "Optical Temperature Probe Employing Rare Earth Absorption." Snitzer et al. disclose a device which used the temperature dependent absorption properties of selected rare earths for sensing temperature. The transmission of light through a rare earth-doped material is monitored, and temperature is related to the amount of light transmitted through the material.